Earth moving equipment, such as a motor grader for example, often operates in environments with poor underfoot conditions. Some earth moving machines are equipped with an all-wheel drive system so that front wheels of the machine, normally used for steering, can also be engaged and driven to help propel the machine. Normally, only rear wheels are driven, but when poor underfoot conditions are encountered causing excessive wheel slip, the front wheels of the machine can also be driven to increase traction of the machine, and thereby reducing slip and maintaining directional control in the machine.
For some earth moving equipment like a motor grader, such all-wheel drive system, typically, includes a mechanical drivetrain for the rear wheels and a hydrostatic drivetrain for engaging the front wheels. However, it is generally known that the hydrostatic drivetrain is inherently less efficient than the mechanical drivetrain, in an all-wheel drive system application. Parasitic loads due to inefficiency of the hydrostatic drive, in combination with the poor underfoot conditions, may affect a rimpull performance of the machine, when the hydrostatic drive is enabled.
US Patent Publication Number 20130013158, hereinafter referred to as the '158 publication, relates to a control arrangement for vehicles, in particular for motor graders. The control arrangement provides a drive motor, driven rear wheels coupled to the drive motor and further wheels, connectable via allocated hydraulic motors and which are activated by a hydraulic pump coupled to a drive motor and has an adjustable feed volume. The '158 publication provides that the control arrangement includes a sensor to detect rotational speed of the rear wheels and a sensor to detect the rotational speed of the drive motor. The control arrangement further includes an electronic control device configured to receive the rotational speed signals of the rear wheels and the drive motor and which controls the hydraulic motors as a function of the rotational speed. The '158 publication further provides that the hydraulic pump and the hydraulic motors can be activated electrically and adjusted in a continuously variable fashion and the hydraulic pump is connected directly, without valves, to the hydraulic motors in parallel by hydraulic lines. Further, a control device controls the respective displacement volume of the hydraulic motors only as a function of the rotational speed signals of the sensors.
With such an arrangement, the control arrangement of the '158 publication tries to control the individual distribution of displacement values for the hydraulic motors and the hydraulic pump, and to enable connection and disconnection of a hydrostatic auxiliary drive while the machine is in motion. However, the '158 publication does not consider any losses that are incurred by the drive motor when the hydrostatic auxiliary drive is connected, and neither does the '158 publication considers the reduction in a rimpull performance of the machine when the hydrostatic auxiliary drive is connected.